In many applications of RF transmitters, it is desirable to introduce the possibility of the RF transmitter transmitting at different power levels. This is e.g. the case in mobile radio communications, where the output radio power of the mobile stations as well as of the base stations should advantageously be variable depending on signal quality and distance between transmitter and receiver.
If the change in output power level is effectuated simply by changing the input power level to the power amplifier of the RF transmitter, the power amplifier will, at some power levels, have to work at combinations of power supply voltage and input power level which yields poor efficiency of the power amplifier. The power supply voltage can be adjusted and the efficiency of the power amplifier optimised for the highest of the power levels, while the efficiency will be impaired as the input power level to the power amplifier is lowered. Since in many applications of RF transmitters, such as in mobile stations, one has to rely on a battery as the power supply, the battery having limited energy, poor efficiency of the RF transmitter is a severe problem.
This problem has previously been solved by introducing a settable switched power supply to the power amplifier, so that the power supply voltage can be lowered as the input power level to the power amplifier is lowered, the efficiency of the power amplifier thus staying at a high level. Another solution that has been found is to introduce a transformer with switched steps at the output of the power amplifier, so that, without changing the input power level tot he power amplifier, the output signal power can be varied as different parts of the transformer is connected to the transmitter output at different times. However, there are certain drawbacks with each of these solutions. A settable switched power supply generates disturbances which interferes with the RF signal to be transmitted. Further, it is difficult to design transformers for ultra high frequencies covering large frequency ranges. As many RF transmitters today are designed for large frequency ranges, as in e.g. mobile radio telephony, where transmitters often are designed to operate in more than one frequency band, the frequency bands being far apart, the transformer solution is not optimal. Thus, it is desirable to find a transmitter which can transmit RF signals at different power levels in a wide frequency range without impairing the efficiency as the output power is changed, that does not introduce disturbances to the RF signal as the output power is changed.